How Desalination Works

There are many different techniques for desalination. Among the main ones are the multistage flash (MSF) process and reverse osmosis. Countries that do not have anabundance of fresh water are prime candidates for the use of this technology. Quatar’s installations include the Ras Abu Fontas B power and desalination plant, which cost $1 billion to build. Dubai in the United Arab Emirates (UAE) has a 60- million-gal/day desalination plant at Jebel Ali. The plant’s eight MSF units are part of a cogeneration power facility. The fresh water would have been at least as much incentive in the Middle East as the increased total thermal efficiency. The desalination equipment in both cases was supplied by Weir Westgarth (WW).



WW designed the MSF process. The principle of the system is simple: Water and steam in a closed system can be made to boil at temperatures lower than at standard temperature and pressure by reduction of the system pressure. MSF plants contain a series of closed chambers—as many as 20—each held at a lower pressure than the preceding one.

Heated salt water is passed through the overall system. Some of the salt water in each chamber vaporizes into steam. Moisture droplet separators remove salt water droplets. The steam condenses to fresh water when faced with cold tubes and is collected for storage. The last chamber’s brine is quite cool, and it is, in fact, used as the coolant fluid. Then it starts to pick up the latent heat of condensation and increases in temperature. Only a small amount of additional heat is required to prepare this steam for entry into the first flash chamber. One source of this steam is low-pressure steam from a power station.



The key to the reverse osmosis (RO) process is a suitable semipermeable membrane. Improvements in membrane technology now mean that the process can apply to industrial-scale plants. Common contemporary membrane selections are made of cellulose-based polymer or a polyamide layer applied to a microporous polymer film. This membrane is bonded to a porous polyester sheet for structural stiffness. This composite is rolled into a spiral. Spun hollow fine fibers are the finished product. The semipermeable layer is on the outside of the fibers. The total thickness of the composite is about 24 mm. The outside diameter of the tube is about 95 mm, making for a large surface area for rejecting salt. The fibers are made into bundles that are sealed with epoxy in a fiberglass pressure container.